High energy tape having coercivities up to about 750 oersteds, cannot be adequately erased with commercially available ac bulk degaussers so that there is a wide safety margin against attempts to recover the erased signals. Acceptable erasure for such a high energy tape has been accepted to mean that the residual level for the fundamental signal and all harmonics shall be a minimum of 90 dB below the saturated signal level.
Commercially available degaussers employ a coil to which is applied ac, resulting in the generation of a strong ac erase field. These devices may also slowly rotate the reel with respect to the field. It has been found that a 60 Hz, 5000 gauss erase field will only reduce a test signal on high energy tape by 70 dB, "Erasure of Signals on Magnetic Recording Media", Manly, T-MAG 76, Nov. pp. 758-760. This extent of erasure is insufficient to safely provide against recovery of the erased signals.
An erasing technique was developed in which a strong dc magnetic field was first applied to the tape, followed by application of an ac field. Based on this technique, it was proposed to construct a bulk degausser with a 100 dB erasure capability and a continuous duty operation to automatically erase a number of reels of high energy tape. This proposed degausser provides for the application of a strong dc field in the longitudinal orientation followed by moderate ac fields including longitudinal, perpendicular and transverse components. This degausser contemplated the use of a coil which is selectively excited by first a dc power supply and second by an ac power supply.